The purpose of this study is to determine the usefulness of a new breath test (13C-Xylose) for diagnosis of small bowel bacterial overgrowth in children. Breath test results of children with duodenal culture-proven bacterial overgrowth and control children will be compared. The 13C-Xylose breath test will also be compared to other indirect methods of diagnosing bacterial overgrowth, such as baseline breath H2, and serum and duodenal bile acids.